1. Field of the Invention
The present invention relates to a magnetic recording head used for thermally-assisted magnetic recording in which a magnetic recording medium is irradiated with light, thereby anisotropic magnetic field of the medium is lowered, thus data can be written. Further, the present invention relates to a head gimbal assembly (HGA) provided with the head, and to a magnetic recording apparatus provided with the HGA.
2. Description of the Related Art
As the recording density of a magnetic recording apparatus, as represented by a magnetic disk apparatus, becomes higher, further improvement has been required in the performance of a thin-film magnetic head and a magnetic recording medium. Especially, in the magnetic recording medium, it is necessary to decrease the size of magnetic microparticles that constitute the magnetic recording layer of the medium, and to reduce irregularity in the boundary of record bit in order to improve the recording density. However, the decrease in size of the magnetic microparticles raises a problem of degradation in thermal stability of the magnetization due to the decrease in volume. As a measure against the thermal stability problem, it may be possible to increase the magnetic anisotropy energy KU of the magnetic microparticles. However, the increase in energy KU causes the increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. As a result, the head cannot write data to the magnetic recording medium when the anisotropic magnetic field of the medium exceeds the write field limit.
Recently, as a method for solving the problem of thermal stability, so-called a thermally-assisted magnetic recording technique is proposed. In the technique, a magnetic recording medium formed of a magnetic material with a large energy KU is used so as to stabilize the magnetization, then anisotropic magnetic field of a portion of the medium, where data is to be written, is reduced by heating the portion; just after that, writing is performed by applying write field to the heated portion.
In this thermally-assisted magnetic recording technique, there has been generally used a method in which a magnetic recording medium is irradiated and thus heated with a light such as near-field light. In this case, it is important to form a very minute light spot at a desired position on the magnetic recording medium. However, from the beginning, more significant problem to be solved exists in how the light is to be supplied from a light source to the inside of a head, and specifically, where and how the light source is to be disposed.
As for the supplying of light, for example, U.S. Pat. Nos. 6,567,373 B1, 6,795,380 B2 and Japanese Patent Publication No. 2007-200475A disclose a structure in which light is guided to a desired position by using an optical fiber and a reflection means. Further, US Patent Publication No. 2006/0187564 A1 discloses a structure in which a unit having a heatsink and a laser diode is mounted on the back surface of a slider. And US Patent Publication No. 2008/0056073 A1 discloses that a structure, in which a reflection mirror is monolithically integrated into a laser diode element, is mounted on the back surface of a slider. Further, US Patent Publication No. 2005/0213436 A1 discloses a slider structure that is integrated with a semiconductor laser. And Robert E. Rottmayer et al. “Heat-Assisted Magnetic Recording” IEEE TRANSACTIONS ON MAGNETICS, Vol. 42, No. 10, p. 2417-2421 (2006) discloses a configuration in which a diffraction grating is irradiated with the light emitted from a laser unit provided within a drive apparatus.
Furthermore, US Patent Publication No. 2008/0002298 A1 and U.S. Pat. No. 5,946,281 A disclose heads in which a light source is disposed in an element-integration surface of a slider substrate. In these heads, a surface-emitting laser diode, which is easily disposed in the element-integration surface, is used as a light source, and laser light from the surface-emitting laser diode is guided to a desired position by using a diffraction grating. Conventionally, optical devices, such as a reflection mirror, an optical fiber and a laser diode, have been mounted after a polishing operation in the wafer process of the head manufacturing. On the contrary, in these heads, by forming an optical system in the wafer process and further providing the surface-emitting laser diode in the element-integration surface also in the wafer process, the construction of the optical system is completed in the stage of the wafer process, which makes this construction comparatively facilitated and simplified and allows improvement of mass-productivity.
However, the surface-emitting laser diode used in these documents is a vertical-cavity surface-emitting laser (VCSEL) that is widely used. In a magnetic recording head in which such a surface-emitting laser diode and the diffraction grating are disposed in the element-integration surface as described above, an insufficient laser output power in the surface-emitting laser diode and the degradation in function of the diffraction grating due to fluctuation of the wavelength of the laser light are likely to lead to serious problems.
First, as for the insufficient laser output power, the amount of output of near-field light, required for attaining a recording density exceeding 1 Tbits/in2 in a magnetic disk apparatus for performing the thermally-assisted magnetic recording with use of near-field light, has been approximately 1 mW with a spot diameter of 40 nm or less, according to the estimation by the present inventors using simulation and the like. Moreover, the light use efficiency, which the present inventors estimated for the overall optical system in an expected head structure, has been approximately 2%. Therefore, the output power necessary for the laser diode as a light source is estimated to be 50 mW or more. However, a VCSEL generally has a short cavity length, and the output power is about several mW for general use. Therefore, it is difficult for the use of the VCSEL to meet such a high output power.
Next, as for the degradation in function of the diffraction grating due to fluctuation of the wavelength of the laser light, a diffraction grating has a function of changing a propagation direction of the light. This function is achieved by using a grating having a distance and arrangement designed based on the wavelength of incident light, and is significantly affected by the wavelength of the incident light. Here, since the laser diode mounted on a head is a device formed of a semiconductor material, its wavelength changes according to the change of surrounding temperature. Specifically, the assumed temperature in the environment where a magnetic disk apparatus is used is, for example, about −5 to 60° C. (degrees centigrade), and accordingly the wavelength may vary, for example, in the range of approximately from 5 to 10 nm. Therefore, when such a diffraction grating is used, a serious problem may occur such that the function of the diffraction grating is degraded by the wavelength fluctuation and then the laser light may not reach a desired position.
Furthermore, in a VCSEL, the size of the beam spot near the light-emitting surface is extremely small, for example, approximately 0.5 to 5.0 μm. And the divergence angle of the emitted laser light is rather large. Therefore, for example, it may become difficult to monitor the output of the light emitted from the VCSEL in order to adjust the output. Actually, for monitoring the light output of the VCSEL, a part of the laser light emitted from the VCSEL is taken out, then the part of the laser light is detected by a light detector provided also in the element-integration surface. In the case, in order to avoid greater loss in the amount of light used for the thermal assist, the part of the laser light should not be taken out by using a reflecting mirror or the like until the emitted laser light is diverged to a considerable degree. Therefore, the reflecting mirror or the like has no other choice to be provided in a position far away from the light-emitting surface of the VCSEL toward the element-integration surface. As a result, the light-path length from the VCSEL to the light detector increases, which may cause greater light loss and prevent satisfactory detection. Primarily, the considerably large divergence angle of the laser light emitted from the VCSEL has caused a difficulty in transforming the diverged laser light into a light beam with a minute spot size within the head.